The present invention generally relates to parking brakes and, more particularly, to electrically powered parking brakes with direct tension feedback.
Almost all vehicles have a parking brake mechanism, and most of these vehicles have a hand or foot activated lever for manually actuating the parking brake mechanism. Typically, these levers are attached to a cable that causes displacement of the cable and operation of the attached parking brake mechanism. There is a need to provide an electrically actuated cable mechanism that operates reliably with a minimum number of parts and has a low cost, so that manually actuated mechanism may be replaced or easily selectively replaced.
It is known in the art to provide electric-motor driven brake systems. For example, see U.S. Pat. Nos. 4,281,736, 4,561,527, 4,629,043, 4,795,002, 4,865,165, 5,004,077, 5,180,038, 5,485,764, 5,542,513, and 5,590,744 which each disclose an electric motor-driven brake system, the disclosures of which are expressly incorporated herein in their entirety by reference.
While these electric-motor driven brake systems may adequately actuate the brake mechanisms with varying degrees of success, they are inefficient, typically can be used in conjunction with only one type of cable system, must be mounted within the vehicle, have relatively large package sizes and utilize a large number of components and they are relatively heavy. Additionally, when applying the brakes, the control module is adapted to stop the rotation of the electric motor when a predetermined tension is achieved in the cable. Cable tension is determined by measuring cable travel using a positive encoder or a motor current using a shunt. Both of these devices measure and control cable tension indirectly. However electric motor torque is not constant with temperature, over time cables stretch, and rotating components wear which could lead to the application of insufficient cable tension to the brakes and ultimately park brake failure. Accordingly, there is a need in the art for an improved electric parking brake assembly which measures cable tension directly, has a relative small package size, has a relatively small number of components, is relatively light weight, and overcomes many of the shortcomings of prior art devices.
The present invention provides an electric brake assembly for tensioning a brake cable attached to a brake. The brake assembly, in combination, includes an electric motor with an output shaft and a transmission adjacent to the output shaft. The transmission has a threaded lead screw coupled to the output shaft. Also, a drive nut is adjacent to the lead screw. The drive nut is adapted to move linearly when the lead screw is rotated. The drive nut has a portion forming a bore and a seat. The cable extends through the bore. The cable has an attachment portion. A retainer member is coupled to the attachment portion. A housing member is disposed in the bore to urge the retainer member away from the seat. Additionally, a tension feedback device is adjacent to the attachment point. The tension feedback device is adaptable to determine the displacement of the cable toward the seat when the output shaft is rotated and is adapted to stop the rotation of the output shaft when the cable moves a predetermined distance.
In accordance with another aspect, the present invention, a tension device is adapted for controlling the tension load in a cable that is actuated by an electric motor having an output shaft and a transmission coupled to the output shaft and including a lead screw. The tension device, in combination, includes a drive nut coupled to the lead screw. The drive nut is adapted to move linearly when the lead screw is rotated. The drive nut has a bore, counter bore and a ledge between the bore and counter bore. The cable extends through the bore. The cable has an attachment portion. A retainer is coupled to the attachment portion. A biasing member is disposed in the counter bore and urges the retainer away from the ledge. Additionally, a tension feedback device is adjacent to the retainer. The tension feedback device is adapted to measure the displacement of the cable toward the ledge when the electric motor rotates the lead screw and is adapted to stop the rotation of the electric motor when the cable is displaced a predetermined distance.
According to yet another aspect of the present invention, a tension-sensing device is adapted for measuring the load on a cable attached to a brake. The device, in combination, includes an electric motor having an output shaft. A control module is coupled to the electric motor for controlling the rotation of the output shaft. The transmission is adjacent to the output shaft. The transmission has a threaded lead screw coupled to the output shaft in rotation therewith. A drive nut is adjacent to the lead screw. The drive nut moves linearly along the lead screw when the lead screw is rotated. The drive nut has a portion forming a bore and a seat. One end of the cable extends through the bore. An attachment portion is on one end of the cable. A retainer member is coupled to the attachment portion. A biasing member is disposed in the bore and urges the retainer member away from the seat. Additionally, a tension feedback device is adjacent to the retainer. The tension feedback device is adapted to measure the displacement of the cable toward the seat when the electric motor rotates the lead screw and is further adapted to provide a signal to the controller when a predetermined distance forms between the feedback device and retainer in order to stop the rotation of the electric motor.
An advantage of the present system is that cable tension load is determined by the displacement of the cable relative and the biasing member as measured by a tension feedback device. This eliminates cable tension load variability due to temperature effects, cable stretch, and wear of rotating mechanical components.
From the foregoing disclosure and the following more detailed description of various preferred embodiments it will be apparent to those skilled in the art that the present invention provides a significant advance in the technology of electric parking brakes. Particularly significant in this regard is the potential the invention affords for providing a high quality, reliable, light weight, small packaged, easily assembled and disassembled, low cost assembly. Additional features and advantages of various preferred embodiments will be better understood in view of the detailed description provided below.